1. Field of the Invention
The present invention relates to an image forming apparatus which transfers a visible image having a plurality of colors onto a transfer material which is carried by an endless-belt-type transfer material conveying member or onto an endless-belt-type intermediate transfer member to form a multicolor image.
2. Description of the Related Art
Conventionally, there is known an image forming apparatus which emits light modulated by an image signal to form an electrostatic latent image on a photosensitive drum serving as an image carrier, develops the electrostatic latent image with developers of respective colors to form a toner image of the electrostatic latent image, and transfers the toner image onto a transfer material directly or through an intermediate transfer belt. This apparatus is referred to as a single-drum type color image forming apparatus.
Also, there is known an image forming apparatus having four image forming sections, each section including a photosensitive drum associated with one of four colors and an electrophotographic process unit disposed in the periphery of the photosensitive drum. Toner images formed by the respective image forming sections are transferred onto a transfer material directly or through an intermediate transfer belt. This apparatus is referred to as a multi-drum type color image forming apparatus.
Since the multi-drum type color image forming apparatus transfers images in a multiplexed manner, there is a concern about the so-called “color misregistration”. The color misregistration occurs when color images formed on the respective photosensitive drums become out of registration on the transfer material due to various reasons, such as a mechanical mounting error between the photosensitive drums, an optical path length error between laser beams, or an optical path variation between laser beams.
A technique generally employed to correct color misregistration includes forming a color misregistration correction pattern on an intermediate transfer member (intermediate transfer belt) or a transfer material conveying belt, and then detecting the pattern with a photosensor serving as a pattern detection unit arranged adjacent to a photosensitive drum at the most downstream side of the image forming section. In this way, color misregistration is detected based on the color misregistration correction pattern, and an image signal which is to be recorded can be corrected electrically. Also, there is another color misregistration correction technique in which a folding mirror located in a laser beam path is driven to automatically correct a change in an optical path length or a change in an optical path.
The techniques described above relate to a stationary color misregistration with respect to misregistration of a color misregistration correction pattern. Japanese Patent Application Laid-Open No. 10-3188 discusses a positive approach to correcting a variable misregistration or an uneven pitch occurring at certain regular intervals.
To be more specific, (1) a stationary misregistration correction pattern and a periodic misregistration correction pattern are formed so that periodic misregistration and uneven pitch as well as stationary misregistration can be corrected. Also, (2) a rotation variation of a photosensitive drum or a transfer material conveying belt is detected, and the rotation speed of a polygonal mirror is controlled based on the detected speed variation.
However, regarding the aforementioned technique (1), the periodic misregistration is due not only to one body of rotation but to a combination of a plurality of bodies. Thus, even if a periodic misregistration in the photosensitive drum is corrected, misregistration due to other factors cannot be corrected.
Also, regarding the aforementioned technique (2), misregistration is due not only to a speed variation of a body of rotation but also to an eccentricity of a roller which drives the photosensitive drum or the transfer material conveying belt. Accordingly, with regard to the aforementioned technique (2), even if a speed variation of the photosensitive drum or the transfer material conveying belt is corrected, the eccentricity cannot be corrected.